There has been a recent resurgence in the interest in unmanned aerial vehicles (UAVs) for performing a variety of missions where the use of manned flight vehicles is not deemed appropriate, for whatever reason. Such missions include surveillance, reconnaissance, target acquisition and/or designation, data acquisition, communications datalinking, decoy, jamming, harassment, or one-way supply flights. This interest has focused mainly on UAVs having the archetypical airplane configuration, i.e., a fuselage, wings having horizontally mounted engines for translational flight, and an empennage, as opposed to "rotor-type" UAVs, for several reasons.
First, the design, fabrication, and operation of "winged" UAVs is but an extrapolation of the manned vehicle flight art, and therefore, may be accomplished in a relatively straightforward and cost effective manner. In particular, the aerodynamic characteristics of such UAVs are well documented such that the pilotage (flight operation) of such vehicles, whether by remote communications datalinking of commands to the UAV and/or software programming of an on-board flight computer, is relatively simple.
In addition, the range and speed of such UAVs is generally superior to rotor-type UAVs. Moreover, the weight-carrying capacity of such UAVs is generally greater than rotor-type UAVs such that winged UAVs may carry a larger mission payload and/or a larger fuel supply, thereby increasing the vehicle's mission efficiency. These characteristics make winged UAVs more suitable than rotor-type UAVs for certain mission profiles involving endurance, distance, and load capability. Winged UAVs, however, have one glaring deficiency that severely limits their utility.
More specifically, winged UAVs do not have a fixed spatial point "loiter" capability. For optimal performance of many of the typical mission profiles described hereinabove, it is desirable that the UAV have the capability to maintain a fixed spatial frame of reference with respect to static ground points for extended periods of time, e.g., target acquisition. One skilled in the art will appreciate that the flight characteristics of winged UAVs are such that winged UAVs cannot maintain a fixed spatial frame of reference with respect to static ground points, i.e., loiter. Therefore, mission equipment for winged UAVs must include complex, sensitive, and costly motion-compensating means to suitably perform such mission profiles, i.e., maintenance of a constant viewing azimuth for static ground points.
Rotor-type UAVs, in contrast, are aerodynamically suited for such loiter-type mission profiles. The rotors of the main rotor assembly of such UAVs may be operated so that the UAV hovers at a fixed spatial frame of reference with respect to static ground points. Maximum hover efficiency is typically achieved through the use of a shrouded duct configuration. The duct acts to direct the rotor-driven air in an almost purely downward direction, thereby providing aircraft lift.
Prior art ducted rotor-type UAV designs have utilized various structural configurations to shroud the rotors within the fuselage of the aircraft, wherein one such configuration utilizes a toroidal fuselage structure. Toroidal structures are generally closed-cell, continuous ring shapes having inherently high structural stiffness. Examples of such a toroidal fuselage structure are described in commonly-owned U.S. Pat. No. 5,152,478, entitled AN UNMANED FLIGHT VEHICLE INCLUDING COUNTER ROTATING ROTORS POSITIONED WITHIN A TOROIDAL SHROUD AND OPERABLE TO PROVIDE ALL REQUIRED VEHICLE FLIGHT CONTROLS, and commonly-owned U.S. Pat. No. 5,150,857, entitled GEOMETRY FOR UNMANNED AERIAL VEHICLES. In general, prior art ducted rotor-type UAVs have not addressed the design considerations necessary for achieving structural and lightweight efficiency.
One prior art toroidal fuselage structure utilized a substantially flat duct portion with removable C-shaped cover portions attached around its periphery. The cover portions were stiffened through the use of equally spaced bulkhead structures that were riveted thereon. These bulkhead structures were needed to react the loads associated with the high suction profile that exists on the upper surface of the toroidal structure. The flat duct portion consisted of a continuous honeycomb sandwich structure for preventing bending of the duct due to the suction loads and furthermore for supporting mission payload equipment and subassemblies that were mounted directly thereon. Since the duct portion alone lacked sufficient strength to accommodate all flight loads, it was necessary that the cover portions be sufficiently stiff and structurally strong to react loads tending to distort the fuselage contour (in a circumferential direction). The configuration resulted in an extremely heavy solution.
Prior toroidal shroud configurations also employed a variety of strut configurations to attach the rotor assembly to the fuselage. A four strut configuration was thought to be sufficient to transfer rotor loads efficiently, however this combination resulted in severe deflections of the strut members and distortion or "egging" of the toroidal fuselage. The distortion of the toroidal fuselage may result in blade contact with the duct wall. Structural modifications to stiffen the toroidal fuselage add additional weight to the aircraft and, consequently, require more power to operate, a luxury that rotary-type UAVs do not have.
A need, therefore, exists for providing an optimized toroidal fuselage configuration which provides a high bending stiffness and structural strength while maintaining a lightweight design.